1. Field of the Invention
The present invention relates to a field of telescope components, and in particular to a telescopic tube that can be used in a Newtonian reflector.
2. Description of Prior Art
A reflector is one of the three major astronomical telescopes. The reflector utilizes lenses and reflecting mirrors to condense the light, thereby forming an image. The light passes thorough a major condensing reflecting mirror and is sequentially reflected to a second reflecting mirror and an eyepiece. In this way, an observer can utilize the eyepiece to observe a target. The Newtonian reflector is one that is commonly used for an astronomical telescope. It utilizes a reflecting mirror having a concave or spherical surface to condense the light that passes through the lens. Thereafter, the condensed light is reflected to a planar reflecting mirror located at the front end of the tube. Then, the planar reflecting mirror reflects the light into the eyepiece outside the telescope, so that the observer can observe the target. Since the cost of the reflecting mirror used in the reflector is much lesser than that of a lens, and the whole structure of the reflector is simple, it is very suitable to observe some dimmer galaxies or nebulas outside the Galaxy. Sometimes, it is feasible to use this kind of telescope to observe the moon or other planets. Further, the Newtonian reflector, especially the Dobsonians reflector supported by a simple bracket, is very popular for the astronomical observers.
As shown in FIG. 1, the main body of the current Newtonian reflector is constituted of a tube 2, an object lens 4, a reflecting mirror 3 and an eyepiece 1. The bottom end inside the tube is provided with a glass-made object lens having a spherical or parabolic surface coated with a reflective material, which reflects and condenses the parallel lights entering the tube to a point. Then, via the reflecting mirror located on an upper end inside the tube and fixed in front of the focus, which is a small planar mirror, the reflected light from the object lens can be reflected at a right angle into the eyepiece provided on an edge outside the tube, thereby magnifying the image to obtain a picture like that available in a common telescope. The tube is used as a support for each major part in a light path, and in particular, is used to support an object lens of a large weight and a mount for the object lens. Therefore, the tube must have sufficient strength, and is often made from a metal or plastic material that has a high strength and low degree of deformation. The inner diameter of the tube is larger than that of the object lens by 2-3 mm, thereby facilitating the installation and adjustment of the object lens. In general, the length of the tube is at least identical to the focus distance of the objective lens. If the length of the tube is too short, the focus of the major lens may extend beyond the tube by a long distance. Unless the size of a secondary mirror is large enough, some light may lose at the edge of the visual field when the observer chooses a wide-angle eyepiece. In comparison with a reflecting tube, the length of the tube of the Newtonian reflector is only eight times of the diameter of the object lens, which is approximately one half of the reflecting tube. However, for a reflector having a larger diameter of object lens, the length of the tube is still too large. The larger the diameter of the telescope is, the larger the ability of condensing is. As a result, the number of the objects that can be observed is larger. Further, the larger the diameter of the object lens is, the higher the resolution is. Therefore, in order to increase the ability of condensing, the only way is to increase the diameter of the telescope, which in turn has to increase the length of the tube of the telescope. The length of the tube of the current middle-sized Newtonian reflector even achieves 1.0 to 2.0 m. In addition, the brackets for mounting the lens and mirrors also increase the whole weight and volume greatly, which increases the difficulty in transferring the telescope and the accommodating space for the observer. When the observer intends to transfer the telescope to a desired location, such a huge telescope cannot be disposed in a common car. Further, the large weight of the tube also hinders the transferring operation.
Therefore, in view of the above drawbacks of prior art, the inventor of the present invention has proposed a novel and inventive structure based on his expert experience and researches.